1. Field of the Invention
The present invention relates generally to a solder pad structure, and in particular, to a solder pad structure with high bondability to a solder ball.
2. The Prior Arts
Recently, electronic products are developed smaller and smaller. Correspondingly, openings of electronic points on substrates, for example those openings for solder ball pads or bump pads, as well as the solder balls or bumps filled in the openings are also configured smaller and smaller. For example, a pitch of a ball grid array (BGA) solder pad is now configured with a size of 0.4 mm, or even 0.3 mm, comparing with the previous typical sizes thereof about 1 mm. Further, a typical solder mask (SM) solder pad opening is diminished from a previous size about 350 μm to a current size under 200 μm. Similarly, a typical flipchip solder pad opening is also diminished from a size about 100 μm to about 70 μm. However, this trend also causes a great difficulty on the packaging process.
Bondability between a solder ball and a solder pad is generally positive proportional with a contact area of the solder ball and the solder pad. As such, when the sizes of the solder ball and the solder pad become smaller and smaller, the bondability therebetween decreases correspondingly. A lower bondability between the solder ball and the solder pad only causes dropping off, peeling off, crack of the solder ball from the solder pad after being packaged when suffering external forces. This may cause disconnection of electronic points, and opening of the circuit thereof. As such, upon the configuration of small size solder pad and solder ball, how to improve the bondability therebetween becomes an important concern in this art.
For example, electronic components for handheld devices are often required to be tested by dropping test, so as to guarantee the final products will not be damaged due to failures caused by unconscious dropping on the ground. The failures hereby often are cracks happened to the solder balls when impacted by the dropping. An IC chip in an encapsulation process is often processed by implanting a solder ball after being assembled to the substrate. Thereafter, the assembler then disposes the encapsulated component on a PCB solder pad with assembly equipment. A solder ball of the IC chip is melted and then welded to the PCB by a reflow process. In general, solder pads of a PCB often are greater than that of IC substrates, and therefore the problems of reliability of the interfaces between the solder balls and the solder pads are more likely to happen at the IC substrate side. In this manner, when an electronic product is impacted, a crack is often seen at an inter-metallic compound (IMC) layer configured by the IC substrate and the solder pad, for example, typically a cell phone dropped on ground often cannot be turned on.
Generally speaking, in order to obtain a better bondability between the solder ball and the solder pad, a solder resist opening (SRO) is usually enlarged. However, as the entire component is desired to be smaller, the size of the SRO is strictly restricted. Further, an enlarged SRO often requires more precise image transferring and alignment exposure, which requires excessive equipment and higher cost. Furthermore, in order to prevent the IMC layer from being thickened due to the reflow processes, a thin layer of palladium is sometimes introduced between a nickel layer and a gold layer to depress the thickening of the IMC layer. However, the introducing of the palladium layer requires modification of the plating equipment. Moreover, palladium is very expensive metal element, thus the use of palladium increases the production cost.